This invention relates generally to methods and apparatus for computed tomography (CT), and more particularly to methods and apparatus for motion compensation in CT.
Motion compensation is a major issue in computed tomography (CT). This is mainly due to the fact that CT requires more than 180° of projections to formulate a reconstructed image. Given various limitations encountered in the commercially available scanners, the amount of time required to collect a complete set of projections is significant as compared to the patient motion. For illustration, lets consider the imaging of a heart. Cardiac CT is typically performed with the aid of an EKG signal to synchronize the data acquisition and reconstruction with the phase of the cardiac motion. The data needs not only be acquired during the quiescent cardiac period, but also needs to be collected at the same cardiac phase over multiple cardiac cycles. Although EKG gating performs satisfactorily in most cases, there are a significant number of cases in which the gating provided by the EKG is suboptimal. This is mainly due to the fact that EKG represents only the electrical properties of the heart. It is well known that the electrical signal does not truly represent the mechanical state of the heart. In addition, the duration of the quiescent period changes with the patient heart rate. As the heart rate increases, the quiescent period shortens. Therefore, for a scanner with a given rotation speed (e.g., 0.35 s), there is an upper limit on the heart rate in order for EKG-gated CT to function properly. Analysis has shown that the upper limit is around 70 bpm. This represents less than 70% of the patient population. It is desirable to scan patients with higher heart rates. It is also desirable to enable scanners with slow gantry speeds to perform cardiac CT scans.
Earlier, an integrated ultrasound-CT approach was proposed wherein both the ultrasound and CT data are acquired simultaneously during the data acquisition and reconstruction process (see U.S. patent application Ser. No. 11/276,195, titled Combined Ultrasound and CT Device for Motion Compensation). Because the two datasets are acquired at the same time, the information provided by ultrasound on the shape and location of the heart can be used directly to help to combat CT motion artifacts. Below is disclosed a reconstruction algorithm for the motion compensation.